Model type attributes as regular Attrs.

Model type attributes as regular Attrs.

Specifically, AttributedType now tracks a regular attr::Kind rather than
having its own parallel Kind enumeration, and AttributedTypeLoc now
holds an Attr* instead of holding an ad-hoc collection of Attr fields.

Differential Revision: https://reviews.llvm.org/D50526

This reinstates r339623, reverted in r339638, with a fix to not fail
template instantiation if we instantiate a QualType with no associated
type source information and we encounter an AttributedType.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@340215 91177308-0d34-0410-b5e6-96231b3b80d8